1. Field of the Invention
The present invention relates to a solid state imager for use in such video camera apparatuses as the CCD video camera and, more particularly, to an automatic black balance adjusting circuit for adjusting black balances of video signals when incident light to the solid state imager is substantially nil.
2. Description of the Prior Art
Imaging devices utilizing the solid state image detector such as a CCD (charge coupled device) are generally called solid state imagers. Conventional solid state imagers are known sometimes to develop coloring or irregular brightness in black portions of video images that they produce. This phenomenon stems from black imbalance of video signals. The imbalance, leading to image quality deterioration, is attributable to changes in the temperature characteristics of the solid state image detector as well as in the color temperature of incident light to the image detector. One solution to the deterioration in image quality is a black balance adjusting circuit provided in the solid state imager.
FIG. 3 shows a conventional black balance adjusting circuit contained in the solid state imager disclosed in Japanese Patent Laid-Open No. 63-14554. In FIG. 3, a luminance signal Y fed from a matrix circuit 23 is turned to a control signal by an amplifier circuit 30. The control signal is sent via a resistor 31 and an output terminal 32 to a servo circuit, not shown. Given the control signal, the servo circuit drives an iris motor, not shown, for automatic iris control.
Meanwhile, three primary color signals R, G and B are supplied to the matrix circuit 23 which in turn generates the luminance signal Y and color signals I and Q. The signals R and B are controlled for gain by AGC circuits 21 and 22 so as to reach necessary levels with respect to the signal G. After undergoing gain control, the signals R and B become signals R' and B' that are input to the matrix circuit 23.
The process of gain control takes place as follows: The AGC circuit under control of a memory circuit 26 amplifies the signal R into the signal R' for output to an amplifier 24. The amplifier 24 supplies the memory circuit 26 with an error signal representing the error of the signal R' with respect to the signal G. Based on the information stored in it, the memory circuit 26 outputs to the AGC circuit 21 a gain control signal corresponding to the error signal. Likewise, the AGC circuit 22 under control of a memory circuit 27 amplifies the signal B into the signal B' for output to an amplifier 25. The amplifier 25 supplies the memory circuit 25 with an error signal representing the error of the signal B' with respect to the signal G. Based on the information stored in it, the memory circuit 27 outputs to the AGC circuit 22 a gain control signal corresponding to the error signal. In this manner, the signals R and B are turned respectively into the signals R' and B' at required levels with respect to the signal G for gain control.
Setting a switch 34 to the side of a terminal 34b initiates black balance adjustment and updates gain control information. Specifically, when the switch 34 is set to the terminal 34b side for black balance adjustment, a terminal 34c is connected to ground and the servo circuit is supplied via the output terminal 32 with the control signal clamped substantially to ground voltage by a diode 33. The control signal causes the servo circuit to close the iris in order to shut off incident light. After a predetermined time based on the iris operating time has elapsed, a time constant circuit 35 brings terminals K of the memory circuits 26 and 27 to ground potential. The memory circuits 26 and 27 enter write mode. The gain control information in the memory circuit 26 is updated according to the error signal representing the error of the signal R' with respect to the signal G during incident light shutoff. Similarly, the gain control information in the memory circuit 27 is updated based on the error signal representing the error of the signal B' with respect to the signal G during incident light shutoff.
As described, automatic black balance adjustment is carried out conventionally on the basis of the signal levels of the three primary color signals R, G and B in effect upon complete incident light shutoff. It follows that precise detection of the state of complete incident light shutoff is very important in executing the function.
With three-plate CCD video cameras, optical black level signals derived from the CCD's for the respective colors are timed to set pedestal levels of video signals. Black levels of video signals are set according to the levels (i.e., noise levels) of the three primary color signals R, G and B in effect during complete incident light shutoff. These noise levels vary significantly depending on the difference in characteristics between the CCD's as well as on the temperature characteristics such as dark current. Thus it is difficult to detect the state of complete incident light shutoff through attempts to detect such noise levels. As a result, conventional solid state imagers sometimes start automatic black balance adjustment while the iris is being closed halfway.
In executing automatic black balance adjustment, the above-mentioned prior art solid state imager takes advantage of the fact that it takes the automatic iris about 1.5 seconds going from activation to complete light shutoff. That is, automatic black balance adjustment is arranged to be started about two seconds after operation of the switch 34. In other words, the prior art example substitutes the delayed action of the time constant circuit 35 for the detection of complete incident light shutoff based on signal levels. This means that correct black balance adjustment is not always achieved where the automatic iris works poorly or where a manual iris C-mount lens is utilized.